The present invention relates generally to articulated trucks and more particularly, to articulated trucks having a bed for carrying material therein.
Dump trucks have long been known in the art, for use in carrying different types of loads. One such use is in the construction industry for carrying dirt or other construction materials in the dump truck bed. A typical dump truck bed includes a tailgate defining a rear wall of the truck bed. The tailgate retains the material within the bed when the tailgate is in a storage or raised position and permits material to be ejected from the bed as the dump truck bed is raised. U.S. Pat. No. 5,456,521, assigned to the assignee of the present invention, discloses an unloading gate for a dump truck wherein the gate is mounted to one or more rails disposed on the bottom surface of the bed. The unloading gate moves from one end of the bed to the other to scrape and clean the bed after unloading or dumping of the material. The unloading gate in this patent is utilized in conjunction with a conventional dump truck bed which lifts at its forward end closest to the tractor cab and utilizes gravity to dump the material from the dump truck bed.
Applicant has had under development a new material carrying vehicle with a new material ejection system as is described in U.S. patent application Ser. No. 09/160,698, filed Sep. 25, 1998, entitled xe2x80x98Hydraulic System For A Work Machine Having An Ejector Cylinder And A Tailgate Cylinderxe2x80x99, assigned to the assignee of the present invention, which is hereby incorporated in its entirety herein. A variation of the above system is disclosed in U.S. patent application Ser. No. 08/936,128, filed Sep. 24, 1997, entitled xe2x80x98Tailgate Latching Mechanismxe2x80x99, assigned to the assignee of the present invention, and hereby incorporated in its entirety herein.
Such a material eject system has a motor, for example, a hydraulic cylinder, that is connected to an ejector blade having a home position at the front end of the bed. To initiate a material eject operation, the motor moves the ejector blade toward the rear of the vehicle, thereby pushing the material in the bed off of the rear edge of the bed. The ejector blade is then retracted or returned to its home position at the front of the bed. The bed normally includes a tailgate which prevents material from exiting the bed while the material is being transported. During the material eject cycle, the tailgate is moved between its closed and open positions in synchronization with the operation of the ejector blade.
A vehicle having a material eject system has several advantages over traditional dump trucks. For example, the vehicle bed is not raised; and its center of gravity is not elevated as the material is being ejected. Therefore, vehicle stability is not adversely affected by the material ejection process. By maintaining a lower center of gravity, a material ejection system is more suitable for controlled spreading of the material during the ejection process. Further, by not raising the bed, the vehicle is not exposed to overhead obstacles such as trees and power lines. In addition, the profile of the ejector blade closely matches the cross-sectional profile of the bed and therefore, substantially all of the material in the bed is scraped out of the bed during the ejection process. Thus, a vehicle material ejection system is generally more efficient and flexible than the traditional elevating material dumping systems.
A vehicle material ejecting system normally uses a multistage cylinder connected between the movable ejection plate and the frame of the bed. During a material eject cycle, a hydraulic pump powered by the vehicle motor provides hydraulic fluid under pressure to the larger, full end of the multistage cylinder; and fluid is exhausted from the smaller, annular end of the cylinder back to a fluid reservoir or tank. At the end of the eject cycle, to return the ejector blade to its home position, the output of the pump is switched to supply hydraulic fluid to the annular end of the multistage cylinder; and fluid is exhausted from the full end of the cylinder back to the tank. A substantial volume of fluid, for example, 100 liters, may be required to fully extend the cylinder by pressurizing the full end. However, because of the mechanical amplification factor of the multistage cylinder, a much smaller volume of fluid, for example, ten liters, injected into the annular end is capable of fully retracting the cylinder. Therefore, if the same fixed displacement pump is used to supply both ends of the cylinder, during the return stroke, the cylinder segment at the annular end of the cylinder tries to fill very quickly. However, the rate at which oil can be exhausted from the full end of the cylinder is limited, and oil cannot exhaust from the full end at the same rate that the annular end is attempting to fill. The net result is that the cylinder seals are worked harder, oil in the full end is heated and the cylinder may be damaged. Thus, there is a need to improve the reliability of the operation of the ejector blade return cycle. More specifically, there is a need to provide a fluid supply for returning the ejector blade that matches the volume requirements of the cylinder in the return cycle.
The present invention overcomes the foregoing and other shortcomings and drawbacks of material ejecting systems and methods of material ejecting heretofore known. While the invention will be described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. On the contrary, the invention includes all alternatives, modifications and equivalents as may be included within the spirit and scope of the present invention.
In accordance with one embodiment of the present invention, a fluid cylinder control system includes a cylinder with a high volume port and a low volume port that moves between extended and retracted positions. A first pump is connected to a fluid source and selectively provides a first flow rate of fluid to the high volume port to move the cylinder toward the extended position. A second pump is also connected to the fluid source and selectively provides a second flow rate of fluid to the low volume port of the cylinder to move the cylinder toward the retracted position. The second flow rate of fluid is less than the first flow rate of fluid. In one aspect of the invention one or more valves are used to selectively provide the flow rates of fluid to the cylinder. The use of two pumps permits the different volume requirements of the cylinder to be accommodated, thereby providing a more efficient cylinder operation that increases the life of the cylinder.
Another embodiment of the invention is a method of controlling motion of an ejector operatively associated with a bed of a vehicle, the ejector being powered by a cylinder having a high volume port and a low volume port. The method initiates a first flow rate of fluid with a first pump from the fluid source to the high volume port of the cylinder and provides a flow path from the low volume port of the cylinder to the fluid source. The method also initiates a second, lesser flow rate of fluid with a second pump from the fluid source to the low volume port of the cylinder and provides a flow path from the high volume port of the cylinder to the fluid source.